This invention relates to the metal plating of zincated aluminum and, more particularly, to providing an enhanced adhesive and smooth plating by employing an improved double zincating process and preferably, in combination with a unique electroless metal plating procedure utilizing a specially formulated electroless metal plating bath.
Metal plating of aluminum is of considerable commercial interest and one application is the preparation of memory disks which are used in a variety of electronic applications such as computer and data processing systems. Aluminum is the preferred substrate for the disk although other suitable metals may be employed. In general, a relatively thin layer of nonmagnetic electroless nickel is applied on the aluminum followed by a thin layer of a magnetic material such as cobalt. A signal is stored on the disk by magnetizing the cobalt layer to represent the signal at a selected moment in time.
Typical alloys used for memory disks are Aluminum Association Numbers 5086 and 5586. These disks contain magnesium in an amount of about 4% by weight. Generally, the aluminum disks are about 1.25 to 5 mm. thick and contain, by weight, about 4% to 4.90% magnesium, 0.01% to 0.40% copper, 0.01% to 0.40% zinc, chromium, nickel, iron, silicon and the balance aluminum and inevitable impurities.
The completed metal plated disk must be extremely smooth and uniform so as to prevent "crashing" against the magnetizing head of the device which flies extremely close (generally 5-8 microinches) to the disk surface. While the starting aluminum substrate must itself be extremely smooth and flat as described in U.S. Pat. No. 4,825,680, the metal plating of the disk must likewise be smooth and uniform so that the final disk product meets the exacting specifications required of these type products.
Unfortunately, however, metal plating of a substrate, and even electroless metal plating, does not necessarily produce a smooth coating. Plating voids, inclusions, bridging and the like are only some of the plating problems which can cause a rough disk surface which is unacceptable.
Aluminum and its alloys also present additional plating problems because of the rapidity with which they form an oxide coating when exposed to air. As a result, special treatments must be employed when plating on aluminum. These treatments include mechanical treatments; chemical etches, especially acid etches containing iron, nickel, and manganese salts; alkaline displacement solutions, especially those depositing zinc, brass, and copper; anodizing, especially in phosphoric, sulfuric or chromic acids; and electroplating with zinc at low current densities for a few seconds. Of these treatments, the alkaline displacement solutions are generally the most successful commercially.
While many metals can be deposited on aluminum by displacement, zinc is the most common. In this case, the process is known as the zincate process.
During the years a number of improvements have been made in the conventional zincate formulation and zincating process, with most of them aimed at accelerating the rate of film formation, and the degree of adhesion and uniformity of the zinc coating produced. A detailed summary of the zincating process may be found in Loch, U.S. Pat. No. 4,346,128, and Saubestre, U.S. Pat. No. 3,216,835, which patents are hereby incorporated by reference.
In the conventional zincating process, the aluminum is prepared by alkaline cleaning to remove organic and inorganic surface contaminations such as oil and grease, followed by a cold water rinse. The cleaned aluminum is then sufficiently etched to eliminate solid impurities and alloying constituents which might create voids resulting in bridging of subsequent deposits. After a water rinse, the aluminum is de-smutted to remove metallic residues and aluminum oxides still remaining on the surface. Thorough rinsing is required and then the zincate coating is applied using an immersion zinc bath to prevent re-oxidation of the cleaned surface.
The zinc coating is obtained by immersion of the aluminum part in an alkaline solution containing zincate ions. The amount of zinc deposited is actually very small and depends on the time and type of immersion bath used, the aluminum alloy, temperature of the solution and the pretreatment process. The zinc coating bath also functions as an etching solution and any oxides reformed during the transferring operations are dissolved by the alkaline zincate while depositing zinc onto the aluminum.
The general procedure now followed by industry is to double zincate whereby the first zinc film is removed using nitric acid followed by application of a second immersion zinc deposit. Double zincating is a preferred method for plating aluminum and is especially useful on certain difficult-to-plate aluminum alloys to ensure better adhesion of the final metal layer deposit.
Despite the acceptance and effectiveness of the double zincating process, the need still exists for an improved process providing both enhanced adhesion and smoothness of the metal plating on the zincated aluminum substrate. Without being limited to theory, it is believed that the properties of the metal plate are directly related to the thickness, uniformity and continuity of the zincate coating with thinner coatings generally providing a smoother and more adhesive metal plating.
It is an object of the present invention to provide a method for preparing aluminum substrate articles having extremely smooth metal plated coatings.
It is a further object of the present invention to provide an improved double zincating process for the metal plating of aluminum, which improved process provides a thinner more uniform and continuous zincate coating and produces enhanced adhesive metal plating deposits and metal plating smoothness.
Another object of the invention is to provide an improved electroless metal plating composition and plating method for coating zincated aluminum substrates with extremely smooth coatings.
Other objects and advantages will become apparent from the following detailed description.